Structural engineers often encounter the challenge of specifying truss girders when developing a structure’s load path during the creation of the structural framing plan. A deeper understanding of their functions, placement, and design considerations is crucial for avoiding costly waste, rework, and miscommunication with downstream manufacturers and installers. Let’s dive into the world of truss girders and explore their critical aspects.
The Role and Structure of Truss Girders
Defining a Girder Truss: A girder truss is a truss that supports other trusses. It ranges from 1-5 plies in thickness, with the additional plies lending extra strength and additional material for connecting fasteners to penetrate.
Nailing and Ply Requirements: For a girder supporting trusses with hangers, a minimum of 2 plies is often necessary. This requirement stems from the nailing needs for hangers, typically rated for nails penetrating 3 inches of material.
Optimal Placement of Girders
Hip Ends and Wall Junctions: You’ll commonly find girders at hip ends or where a wall changes direction. In corner scenarios, the girder usually resides on the “outside” of the adjoining bearing. This placement ensures the trusses it supports have a consistent span and design as the adjacent run of trusses, rather than requiring shorter carried trusses.
Residential Home Considerations: In residential settings, a hip girder setback of 6 feet or less from the end wall is ideal. This allows for toenailing the jack trusses onto the hip end, saving time and resources compared to more expensive (and more expensive to install) connecting hardware. More complex houses may require varied hip girder setbacks to accommodate consistent truss to truss spacings.
Vaulted Ceilings and Roof Openings: Girders are also crucial around changes in ceiling profiles, such as vaulted ceilings running perpendicular to the main ridge line. Additionally, they play a vital role around roof openings like chimneys or in supporting the second floor of a building.
Decision Factors for Girder Placement
Shortest Girder Span Preference: Some structures have ambiguity in the optimal location to place a girder. Given a choice, the shortest girder span is often preferable for ease of installation and cost-effectiveness. However, sometimes the building design dictates girder placement, especially to avoid large point loads on structural elements like garage door headers.
Floor Girders vs. Roof Girders: While similar in placement, floor girders have their unique constraints. They can’t carry as much weight as roof girders and are limited to a maximum of 2 plies (due to limitations of fasteners to connect these wider trusses). In certain deep floor scenarios, a rectangular roof-style girder can be used instead due to its higher weight capacity. (Roof-style trusses have the lumber oriented on edge, whereas Floor-style trusses have the lumber oriented flat-wise).
Practical Approaches to Girder Design
Stair Opening Considerations: Also, don’t forget about floor girders around stair openings. For stair openings, a practical approach to developing the loading involves taking half the length of the stair run, multiplying it by the floor load, and then applying that as a distributed load to the supporting girder.
Conclusion: The Art of Truss Girder Design
Specifying truss girders is a blend of technical knowledge, practical experience, and adaptability to the unique demands of each building. Understanding the nuances of their placement and structural requirements is key to efficient and effective structural engineering. Ultimately, the location and size of your girder trusses depend on a wide variety of parameters depending on the building. Let Truss Pal help you develop your structural framing plans for your project. Truss Pal can provide full truss placement diagrams as well as material take offs and IFC models for a more complete picture of your structural framing and coordination needs.